1. Field of the Invention
The present invention relates to a gain amplifier, more particularly, to a high linearity programmable gain amplifier using a switch.
2. Description of the Background Art
FIG. 1 shows a circuit diagram of a discrete step gain switch amplifier (DSGSA) using a prior art switch.
As shown in FIG. 1, a conventional amplifier using a switch consists of an attenuating portion 110 and an amplifying portion 120.
The attenuating portion 110 includes an input terminal RFin, a plurality of switches SW11 to SW14, and a plurality of resistors R11 to R16.
The amplifying portion 120 is composed of an amplifier 121, resistors RF1 and RF2, and an output terminal RFout.
The output terminal of the attenuating portion 110 is connected with the input terminal of the amplifying portion 120.
When all of switches SW11 to SW14 in the attenuating portion 110 are opened, even if an input signal is applied to the attenuating portion 110, no signal is to be amplified by the amplifying portion 120 because any circuit cannot be formed.
Meanwhile, when the switch SW11 of the attenuating portion 110 is closed to form the circuit, an input signal is applied to a first input terminal of the amplifying portion 120, and a signal attenuated by the attenuating portion 110 based on a combined resistance value of resistors R11 to R16 is applied to a second input terminal of the amplifying portion 120.
The turn-on or turn-off of switches SW11 to SW14 may alter the gain of the amplifier 121.
If the attenuating portion 110 exists in the first and second input terminals of the amplifying portion 120, when an amplifying operation in the amplifier 121 is performed in a low gain, the effect of the noise figure from the amplifier 121 is minimal, but, when the amplifying or converting operation in the amplifier 121 is performed in a high gain, the noise due to the switching noise from a switching element or a thermal noise in the amplifier 121, etc, can be added to an original signal, thereby deteriorating the noise figure.
FIG. 2 shows a circuit diagram of a variable gain low noise amplifier constructed by a parallel connection of prior art amplifier.
As shown, the variable gain low noise amplifier has a first selective portion to a fourth selective portion 201, 202, 203 and 204, a power supply and an input/output terminal.
The first selective portion 201 comprises a driving part including resistors R21 and R22, a capacitor C11 and a transistor M11, and further comprises a capacitor C12 and a transistor M12.
The second selective portion 202 comprises a driving part including resistors R23 and R24, a capacitor C21 and a transistor M21 and an amplifying part including capacitors C12 and C23 and a transistor M22.
The third selective portion 203 comprises a driving part including resistors R25 and R26, a capacitor C31 and a transistor M31 and an amplifying part including capacitors C32 and C33 and a transistor M32.
The fourth selective portion 204 comprises a driving part including resistors R27 and R28, a capacitor C41 and a transistor M41 and an amplifying part including capacitors C42 and C43 and a transistor M42.
The operation of the variable gain low noise amplifier will be described below in detail.
If a current I1 is applied to a base of the transistor M12 to select the first selective portion 201, a current between an emitter and a base of the transistor M12 is amplified rapidly and thereby conducting the transistor M12. This also leads the conduction of the transistor M11. As a result, a gain is determined by the capacitor C12.
If a current I2 flows into a base of the transistor M22, the transistor M22 is conducted and conduction of the transistor M22 also leads to conduction of the transistor M21. As a result, a gain is determined by the capacitance ratio of the capacitor C22 to the capacitor C23.
The third selective portion 203 and the fourth selective portion 204 operate in similar manner with the first selective portion 201 and the second selective portion 202.
In conclusion, the transistor M12 does not attenuate a signal, but the transistors M22, M32 and M42 attenuate a signal with gains determined by the capacitance ratio of the capacitor C22 to the capacitor C23, the capacitance ratio of the capacitor C32 to the capacitor C33 and the capacitance ratio of the capacitor 42 to the capacitor C43, respectively.
Accordingly, the total gain of the whole amplifier circuit is changed depending on which selective portion among the selective portions 201 to 204 is selected.
That is, a desired gain of the amplifier circuit is achieved by connecting the selective portions 201 to 204 in parallel and selecting any one of the selective portions 201 to 204.
However, the conventional variable gain low noise amplifier is disadvantageous in that the parallel connection of the amplifying parts, i.e. the selective portions, and application of the electric current to select any one amplifying part among the amplifying parts lead to deterioration of the characteristics of the circuit in view of the bandwidth, and further the application of an electric current to the amplifying parts increases the total power consumption.